In foundry operations and within the field of metallurgy in general, the need to heat great quantities of molten metal, or to maintain the temperature level of quantities of molten metal over a long period of time, often arises since the entire quantity of the melt is not needed immediately after having been melted. There are known designs of smelting and/or temperature-maintenance furnaces for heating or maintaining the temperature of a cast iron melt, as for example, submerged channel induction furnaces and crucible induction furnaces.
These commercial smelting and/or temperature maintenance furnaces cannot be used for maintaining the temperature of liquid molten metals which have previously been treated with particular additives. This is due, in particular, to the fact that the material added escapes from the molten metal during the course of temperature maintenance, which results in so-called decaying of the melt. An example of this would be the cast iron with globular graphite which is currently being produced in increasing quantities using magnesium, and particularly pure magnesium, as an additive. The procedure of treating molten cast iron with pure magnesium to produce nodular cast iron has taken on increasing significance because it is possible by the immersion converter process to add the magnesium to the initial iron melt accurately and extremely economically. Heretofore, however, it has been necessary to pour the liquid molten metal immediately following a magnesium treatment in order to avoid the before-mentioned decaying effect. This, however, is contrary to economic operation of a foundry, since it is extremely desirable to be able to store the molten metal at a desired pouring temperature, after an immersion treatment has been completed, for long periods of time so that treated molten metal with the same characteristics could be available for pouring as the need for it arose.
It is, therefore, the aim of this invention to provide an improved apparatus and a method of use to satisfy the outlined objectives in the easiest possible way whereby molten metal can be stored and its temperature maintained without decay after it has been treated with particular additives such as, for example, nodular cast iron which has been treated with pure magnesium.
In order to meet this and other objectives, the apparatus for storing and/or increasing or maintaining the temperature level of molten metals includes a furnace chamber provided with a pressure-tight cover and an inlet for a pressurized medium. With such an apparatus, a nodular cast iron melt which has been treated with magnesium, and with pure magnesium in particular, can be kept heated to the desired casting temperature for an almost unlimited time. A decrease in the magnesium content or decaying of the nodular cast iron is virtually eliminated. The process herein described meets the objective of providing an economical process of treating a large quantity of an initial iron melt with the respective additives and maintaining its temperature thereafter in a temperature maintenance furnace from which molten metal can be removed in amounts occasionally necessary for casting. It is believed economically practical to modify various known temperature maintenance furnaces, such as the submerged channel induction furnace, so that they, as modified, can be used to carry out the process of this invention.
The inlet and outlet means of the apparatus, in the preferred embodiment of the invention, are provided by a common inlet/outlet siphon, which can also be sealed pressure-tight by using a closing cover in a manner similar to the way in which the furnace chamber is closed. A cover for the siphon may not be necessary for some uses to which the apparatus is put, since only a small loss of magnesium occurs when using an uncovered combined inlet and outlet siphon. Where the highest possible quality is required, the use of a cover on the siphon is recommended.
The pressure-tight sealing of the furnace chamber cover can be accomplished by means of a sealing strip. The sealing strip can be located on the cover of the furnace chamber for sealing contact with the top rim of the furnace chamber. Similarly, a sealing strip may be placed on the closing cover for the siphon in sealing relation to the pouring end of the common inlet and outlet siphon. The sealing strip locks into a sealing channel provided on the confronting walls of the furance chamber or, as the case may be, on the siphon. The sealing strip preferably has a T-shaped cross-sectional profile.
Apparatus carrying out this invention preferably includes a pressure relief valve for the furnace chamber which releases pressure in the chamber through use of known electrical control systems. The cover preferably remains locked until the pressure within the furnace chamber returns to normal atmospheric pressure. A medium under pressure is delivered by way of a supply opening or inlet to the furnace chamber, which can be accomplished using known electrical systems, after a secure pressure-tight sealing of the furnace chamber has been accomplished. Depending on the requirements for use, it may be desirable to provide a second pressure relief valve.
The apparatus is preferably designed as a tiltable electrical induction furnace whereby the inductor is arranged on the lower section of the furnace chamber as perpendicular as possible beneath the furnace. This perpendicular arrangement beneath the furnace is advantageous particularly in that it reduces unwanted slag suspension deposits.
In this invention, the use of an inert gas under pressure is preferred for the medium, and nitrogen and argon have proven most successful. Depending on the output values (for example, magnesium content) desired for the molten metal which is being kept hot, the inert gas with a pressure level of up to six times atmospheric pressure is delivered to the inside of the furnace. The pressure load of the furnace chamber is, in the case of nodular cast iron melt which has been treated with pure magnesium, regulated in such a way that the level of disintegration of the magnesium which is in the melt is kept at approximately zero.